Urea nitrogen salvage (UNS) diverts nitrogen waste into tissue protein through intestinal microbial breakdown and subsequent hepatic amino acid synthesis in conjunction with host harvest of microbial protein. This thesis investigates the contribution of UNS to nitrogen conservation in a fast adapted hindgut fermenting mammal, the Wyoming ground squirrel (Spermophilus elegans), which experiences seasonal extremes in dietary nitrogen input associated with winter hibernation. This thesis examines the role of UNS and the regulating mechanisms that contribute to endogenous protein conservation in groups of Wyoming ground squirrels on variable dietary protein intakes (5%, 20%, and 40%), a two week euthermic fast, and a prolonged period of hibernation. Chapter 1 investigates the capacity for UNS through the contribution of labeled urea nitrogen to the maintenance of body tissues in squirrels experiencing protein challenges after interperitoneal administration of [superscript 15]N2-urea. The resulting urea [superscript 15]N found in skeletal muscle (gastrocnemius), smooth muscle (ileum), and liver tissue suggests that squirrels meeting or in excess of protein requirements had no appreciable utilization of exogenous urea nitrogen. However, squirrels fed a low protein diet, fasted, or in hibernation showed greater levels of [superscript 15]N in all tissues than natural enrichments. These results suggest that the exogenous urea was retained, transported into the gastrointestinal tract (GIT), hydrolyzed by GIT microbes, and the resulting labeled urea nitrogen was used to synthesize host tissues proteins. Levels of [superscript 15]N were greatest for squirrels in hibernation even beyond that of squirrels in a euthermic fast, suggesting a difference in UNS capacity or regulation during a hibernation fast compared to that in a euthermic fast. Chapter 2 investigates three hypotheses examining the possible regulatory mechanisms which may lead to enhanced UNS due to changes in protein intake or hibernation. These three mechanisms include ureolytic microbial capacity, urea substrate availability, and the efficiency of urea hydrolysis. The in vitro urease assays suggest that the fasted and hibernating ground squirrels had lower overall gut urease activities which may indicate smaller populations of urease producing bacteria within gut digesta than those found in the fed squirrels. Therefore ureolytic microbe population may not explain the enhanced urea nitrogen tissue levels found in Chapter 1. Because the urease assay does not describe urea hydrolysis in vivo conditions, a decline in ureolytic microbial population does not necessarily represent decreased in vivo urea hydrolysis. Urea assays determined that there was greater available urea substrate within the GIT of fasted squirrels and squirrels in hibernation above those on a standard diet, and it appears that GIT urea content may be more of an explanatory measure of the tissue urea nitrogen data. The hibernation group showed a significant relationship between liver [superscript 15]N enrichments and digesta urea content, suggesting a greater efficiency of hydrolysis in hibernation than in squirrels fed a standard protein diet and fasted squirrels. This higher efficiency may be a function of enhanced urea transport due to upregulation of urea transporters in the urinary tract and GIT tissues. These urea tranporters were investigated in Chapter 3 and urea transporter-B (UT-B) proteins were identified within the kidney, ileum, cecum, colon, and bladder of the Wyoming ground squirrel. Although efforts were made to determine UT-B abundance in these tissues for squirrels experiencing the aforementioned protein challenges, they were relatively unsuccessful and need to be further investigated.